1. Technical Field of the Invention
The present invention relates to systems and methods for random access and backward playback of video frames.
2. Background Art
Video recording has become extremely popular. Traditionally, video tape has been used to record video streams. A video stream is a sequence of frames. In many cases, a VCR (video cassette recorder) is used to playback the video tape. More recently, computers, such as personal computers, have been used to play video streams.
There are various formats of digital video signals. However, popular digital video formats include MPEG (Moving Picture Experts Group) formats. Current and proposed MPEG formats include MPEG-1 (“Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 MBits/s,” ISO/IEC JTC 1 CD IS-11172 (1992)), MPEG-2 (“Generic Coding of Moving Pictures and Associated Audio,” ISO/IEC JTC 1 CD 13818 (1994); and MPEG-4 (“Very Low Bitrate Audio-Visual Coding” Status: ISO/IEC JTC 1/SC 29/WG 11, 3/1999). There are different versions of MPEG-1 and MPEG-2.
Although video streams are typically played forward, techniques have been developed to play them in the backward direction. Frame-by-frame backward playback refers to playing each frame in a backward sequence (rather than skipping some). A difficulty in playing in the backward direction is that decoding a frame may require use of another decoded frame which has not yet been decoded. One solution is to decode only certain independent frames or a limited number of other frames. This technique is not frame-by-frame and is unsatisfactory in many cases because it results in a loss of detail. Another technique is to decode each frame in a group of pictures before they are needed and then play them in use them as needed. However, this is wasteful in that not all of them might be needed. Still another technique is to decode the frames as needed, but to redecode the frames rather than store them in decoded form for further use. This is likewise wasteful because frames may be repeatedly decoded over a short amount of time.
Accordingly, there is a need for an effective technique for frame-by-frame backward playback.